Progress in FY2018 includes the following: We wrote a review of our work, making the case that thermal biology is different between small and large mammals. At ambient environmental temperature (22 C), over one third of energy expenditure in mice is devoted to maintaining core body temperature, largely by brown adipose tissue. To conserve this energy mice can enter a regulated hypothermia, while humans do not. Since humans expend little or no energy specifically to keep warm, mice studied at thermoneutrality (30 C) may be a better model for human energy homeostasis. In mice, two drugs, dinitrophenol, a protonophore, and CL316243, a 3-adrenergic agonist, both increased metabolic rate at thermoneutrality but only CL316243 increased it at 22 C. Mice housed at thermoneutrality also may become more obese than mice at 22 C. The effect of environmental temperature must be understood to ensure applicability of mouse experiments to human obesity. Our interest in hypothermia is intimately tied to BAT, as induction of hypothermia involves complete inactivation of BAT, which recover reactivates it. Thus, studies of drugs causing hypothermia should interact in the neural pathways that contribute to the regulation and control of BAT.